Electronic film development, also known as digital development, is a method of digitizing color film during the development process as disclosed in U.S. Pat. No. 5,519,510 issued to the present inventor. Conversion of analog images into digital data, or scanning, has become widespread for a variety of uses, including storing, manipulating, transmitting, displaying or printing copies of the images.
In order to convert a photographic image into a digital image, the film image frame is transported through a film scanning station, and illuminated along each scan line with a linear light beam of uniform, diffuse illumination, typically produced by a light integrating cavity or integrator. The light transmitted through the illuminated scan line of the image frame is focused by a lens system on a CCD-array image detector which typically produces three primary color light intensity signals for each image pixel. These light intensity signals are then digitized and stored. Film scanners which enable the electronic development of film have a variety of forms today and the common aspects of film image frame digitizing, particularly line illumination and linear CCD array based digitizers, are described in greater detail in U.S. Pat. No. 5,155,596.
In electronic film development, the developing film is scanned at a certain time interval(s) using infrared light so as not to fog the developing film, and also to increase penetration of the light through any antihalation layers. Some of the incident light is reflected from an emulsion on the film which contains milky, undeveloped silver halide. The undeveloped halide emulsion has a finite depth over which the photons from the light source will scatter and reflect back toward a detector. This depth is within the range of the coherency length of infrared light sources commonly in use in electronic film development today. It is this finite reflective depth which causes noise in the scanned image due to coherency speckle. Noise in the scanned image results in capturing an image distorted by graininess.
Because of the longer wavelength of infrared light, both the wavelength and the dividing fractional bandwidth for a fixed bandwidth contributes to a longer coherency length than normally encountered in visible light. In addition, the width of the milky silver halide layers is very thin in electronic film development, reducing the coherency length necessary to produce interference speckle.
Furthermore, the image seen through the back side of the film is very faint, so any coherency speckle is amplified as the faint image is amplified and the image is distorted. This problem is apparent in scans of the film regardless of whether light is reflected from the top or bottom of the film, or is transmitted through the film. However, it is predominant in the rear reflection scan due to the increased light reflected by the antihalation layer. No prior art methods appear to address this significant problem. Generally, during film processing, the dry emulsion layer over the film substrate is subjected to an aqueous bath which causes the emulsion to expand. During electronic film processing, photons penetrating the emulsion strike particles suspended in the emulsion and reemerge to be registered by light sensors. As the emulsion expands, the distance between the photon reflecting particles varies proportionally. If the resulting difference between the photons' exit paths is a quarter wavelength difference, then a speckle point can change from black to white or from white to black. Thus, any attempt to remove the speckle effect by differencing images made while the emulsion is in a first expanded position and a subsequent second expanded position can actually make the speckle effect worse by overlaying two different speckle patterns. For these reasons, coherency speckle is a significant problem in practicing electronic film development.
To view coherent speckle with the human eye, the path length traveled by the light can be no more than the coherency length of the light source. Beyond the coherency length, the speckle shimmers at the speed of light and appears to the viewer to be continuous. The characteristic grainy, or speckled, appearance of laser light, which is a coherent light source, is due to interference effects which result from coherence. Under laser light, everything in a room appears speckled, and the speckles appear to shimmer as the light, object, or viewer move.
Even under ordinary light, speckle is sometimes seen when there are very short path differences and very narrow light angles involved, as for example when viewing a white sheet of paper in direct sunlight. For noncoherent light, the coherency length is on the order of the wavelength divided by the percent bandwidth. Because this usually amounts only to a few wavelengths of light, coherency shimmer is not normally visible in real world viewing where noncoherent light is the norm.
It is, therefore, an object of this invention to provide a method of electronic film development which significantly reduces noise in capturing a developed or developing image.
It is another object of this invention to provide a method of electronic film development which significantly reduces or entirely eliminates coherent speckle in a developed image.
It is yet another object of the present invention to eliminate noise caused by coherent speckle during electronic film development which is altered by emulsion expansion.
To achieve these and other objects which will become readily apparent upon reading the attached disclosure and appended claims, an improved method of electronic film development which significantly reduces the amount of coherent speckle noise in an image is provided. Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.